Implantable ventricular defibrillators, including multi-programmable, pacemaker/cardioverter/defibrillator (“PCD”), typically employ epicardial or subcutaneous patch electrodes, alone, or in conjunction with one or more transvenously introduced endocardial leads with one or more electrodes disposed within a heart chamber or blood vessel. Ventricular defibrillation is typically effected with at least one electrode extending along an endocardial lead body disposed within the right ventricle and one or more additional defibrillation electrodes disposed outside the right ventricle to provide two or more defibrillation current pathways through the chamber of the heart to be defibrillated. Other endocardial defibrillation leads for transvenously introducing and positioning defibrillation electrodes into the right atrium and/or superior vena cava, the coronary sinus, the right outflow track or other locations in proximity to the heart have been disclosed in the prior art, including commonly assigned U.S. Pat. No. 4,932,407 to Williams.
The typical endocardial lead defibrillation electrode is configured as an elongated wire of high conductivity that is spirally space wound or close wound about the lead body for a length appropriate for the intended use. The spacing of the coil turns retains flexibility of the lead body along the length of the electrode and distributes the electrode surface area along the length thereof. The wire cross-section is typically circular, as shown in U.S. Pat. No. 5,042,143 to Holleman et al., or rectangular, as shown in U.S. Pat. No. 4,481,953 to Gold et al., U.S. Pat. No. 5,090,422 to Dahl et al., and U.S. Pat. No. 5,265,653 to Kroll et al., although other wire configurations, e.g. the wrapped coils of U.S. Pat. No. 5,439,485 to Mar et al., have also been proposed. The coiled wire electrode may be formed of a single wire or in a multi-filar configuration of interlaced wires. The coiled wire turns are typically partially embedded into the underlying lead body insulation to mechanically stabilize the exposed coil turns at the distal portion and direct the defibrillation current outward of the lead body.
When one continuous coil performs as the conductor and the electrode, the exposed electrode portion cannot be distinguished from the insulated portion when the lead is implanted because the entire length of the lead is of equal radiopacity. Thus, it becomes difficult to see with precision where the exposed electrode portion of the coil resides within the heart—the right ventricle, atrium, superior vena cava, etc. One approach has been to ascertain in advance the length of the exposed electrode portion and based on that information estimate the position of the electrode. However, if the anatomy creates a curvilinear path, difficulty is encountered in making the estimate without multiple fluoroscopic views.
Accordingly, there is a need to accurately determine the position of the electrode to strategically place the lead in the heart within a zone, provide effective delivery of electrical charges at the zone and enable selective positioning of the electrode.